Light-emitting diode (LED) devices are useful because they have lower power consumption, long-term durability and lower cost, comparing with conventional incandescent and fluorescent lamps. Therefore, LED devices are widely used in various fields such as displays, traffic lights, data storage apparatus, communication apparatus, lighting apparatus, and medical apparatus.
With the development of high efficiency LEDs that emit blue or ultraviolet (UV) light, it has become feasible to produce a white LED device with a wavelength-converting method. A portion of the primary light emitted from such an LED is converted into the secondary light with a longer wavelength by phosphor. Conversion of the primary light of the LED to a longer wavelength is commonly referred to as “down-conversion” of the primary light. At least a portion of the secondary light combines with the unconverted portion of the primary light to produce white light.
Therefore, various attempts have been made to generate white light by using blue LEDs or UV LEDs. A prior art technology involves ultraviolet (UV) light at a wavelength of about 390 nm, emitted from an LED and converted by red, green, and blue phosphors to create white light. The disadvantage of the UV-phosphor white light system lies in its low durability.
Another prior art technology involves a white light generating system composed of a blue LED, which emits light having a wavelength between 450˜470 nm, together with YAG:Ce phosphor. The system produces white light of a high color temperature ranging from 6000 K to 8000 K, which is comparable to sunlight, and a typical color rendering index (CRI) of about 70 to 75. However, the conventional system has problems such as the large deviation of color temperatures, and the unstable quantum efficiency of the blue LED which decreases rapidly when driven by high current density. Since there is a large deviation of the color temperatures of the LEDs in a wafer, an additional chip sorting process after dicing is necessary for the chips to provide white light LED devices with a stable color temperature, and thereby increases the cost. Another prior art technology involves a lamp that contains at least three LED chips—red, green, and blue chips, encapsulated in a single epoxy package. The chips are powered via at least four electrical leads. The multiple-chip device is complicated and the cost is higher than other white light illumination apparatus.
In view of the above-mentioned, improved light efficiency and color quality is desirable for white light illumination.